Reduced plate height optimum value

V = 4.5. These are optimum values from a graph of reduced plate height versus reduced... 

The curves represent a plot of log (h ) (reduced plate height) against log (v) (reduced velocity) for two very different columns. The lower the curve, the better the column is packed (the lower the minimum reduced plate height). At low velocities, the (B) term (longitudinal diffusion) dominates, and at high velocities the (C) term (resistance to mass transfer in the stationary phase) dominates, as in the Van Deemter equation. The best column efficiency is achieved when the minimum is about 2 particle diameters and thus, log (h ) is about 0.35. The optimum reduced velocity is in the range of 3 to 5 cm/sec., that is log (v) takes values between 0.3 and 0.5. The Knox... 

We note that uopt is of similar magnitude to the transition velocity ve = DJdp (see Eq. 12.22). Since the reduced velocity v is defined as vlve and the reduced plate height h is Hldp (Eq. 12.25), the above optimum conditions (v near ve and H near dp) correspond roughly to unit values of h and v, a conclusion consistent with Figure 12.3. 

Figure 2.4 Relationship between reduced plate height (ft) and reduced velocity (v) according to the Knox equation (ft = + Bjv + Cv) (equation 2.32)). (a) Optimum values for the...

The variation of the reduced plate height (section 1.5.3) as a function of the reduced mobile phase velocity for a packed column and high performance layer is illustrated in Figure 6.4 [1,62]. The optimum reduced mobile phase velocity is shifted to a lower value compared with the column and the minimum in the reduced plate height ( 3.5) is higher than typical values for a good column 2.0-2.5). Also, at higher reduced mobile... 

On the other hand, we have seen in Chapter 3 that for small molecules a reduction of the particle diameter below about 1pm stops making sense because of the pressure requirements and the high linear velocities associated with the optimum of the plate height-velocity curve. However, for larger molecules with lower diffusion coeflBcients, the optimal linear velocity moves to smaller values in direct proportion to the diffusion co dent. This means that the pressure is also reduced by the same factor, if we keep column size and particle size constant Since the pressure constraints are lowered, we can reduce the particle size further, beyond that which made sense for small molecules. So while a good choice of a particle size for a small molecule was around 3pm, it may be around 0.3pm (300nm) for a macromolecule. 

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